1. Field of the Invention
The present invention relates to a wire straightening device which is capable of reliably accomplishing the wire straightening task while being lightweight and compact, and which may be mounted on an arm end of an arc welding robot within the restriction of load weight. The present invention is also concerned with a welding torch incorporating such a wire straightening device.
2. Description of the Prior Art
In the art of carbon dioxide (CO.sub.2) welding, Metal Argon Gas (MAG) welding and/or Metal Inert Gas (MIG) welding, it has been an established technique to continuously feed welding wire to a torch so as to perform the welding operation intended. Welding wires are of two types according to the amount in which they are used, namely those wound and stored in a pail pack and those wound on a reel. Of these two types, the pail pack type wire is wound and stored in a predetermined manner in a pail container comprising an inner and an outer cylinder, and during the welding operation, the wire is continuously drawn from the container to the torch.
In general, welding wires are conditioned to have high tensile strength. However, the wires become distorted during the manufacturing process, particularly when they are wound into a pail container in a coiled configuration. The wires also become spirally twisted when they are drawn from the pail container. As shown in FIG. 1, when such twist remains on a welding wire 14, the tip of the wire 14 extending from a nozzle 12 of a welding torch 10 swings irregularly as the wire is drawn from the nozzle 12, resulting in a zigzag welding bead and/or out-of-centricity on a work to be welded. In manual welding, such swing of a wire tip can be more or less eliminated by the operator monitoring the welding line. However, this requires substantial skill and experience. With automatic welding devices such as arc welding robots and self-travelling welders, the swing of wire at a nozzle tip can lead to fatal defects.
Various wire straightening devices have been devised heretofore to straighten distortion and/or twist of a welding wire drawn from a pail container and eliminate swing of the wire at the nozzle tip. While the prior art devices have generally been successful in straightening welding wires, they are rugged in construction and bulky, and significantly heavy.
When the above-noted CO.sub.2 welding, MAG welding and/or MIG welding are carried out for a large amount of work, for example, in an automobile assembly line or under welding conditions where man power is impractical, computerized, multi-articulated welding robots are generally used. Specifically, as shown in FIG. 2, the arc welding robot includes a robot body 16 with an arm 18 turnably and tiltably secured thereto, and the forward end of the arm 18 has a welding torch 10 serving as a wrist. Thus, wire is drawn from a pail pack 22 as it is straightened by a known straightening device 24 provided on the base portion of the arm 18 and a cooperating wire feeding motor 26 provided downstream of the straightening device 24, to be fed to the forward end of the torch 10 through a flexible conduit 28. In this arrangement, the welding torch 10, which serves as a wrist, is capable of performing three-axes motion including, for example, horizontal turning, vertical turning and vertical swing. The motion of the welding torch 10 relative to the work to be welded is controlled in accordance with the programmed procedures inputted in a control unit (not shown). It will be noted that the hose for supplying shield gas and cable for supplying positive welding current are not shown in FIG. 2.
When the welding operation is performed using the multi-articulated robot as described above, the welding torch serving as a wrist is capable of performing three-axes motion for example, and hence the torch end provides a composite motion derived from each of the three-axes motion. Thus, in the construction shown in FIG. 2, when the welding torch 10 effects the complex, composite motion, the welding wire 14 of which the twist has been removed by the straightening device 24 will again be given a twist and other distortion in the path from the device 24 through the conduit 28 to the torch 10. For this reason, the tip of wire 14 extending from the torch 10 tends to swing irregularly, as mentioned hereinbefore. Specifically, the straightening device 24 mounted on a base portion of the arm 18 does not work on the twist imparted to the wire between the straightening device 24 and the welding torch 10. In the welding robot, therefore, the straightening device should preferably be located immediately before the torch 10, thereby straightening the twist and distortion produced in the path up to the torch.
It will be important to note that in the multi-articulated robot, load weight which may be supported by the wrist is limited. If the load weight exceeds the limit, accurate and smooth motion may not be attained. Therefore, from an idealistic viewpoint, the straightening device should be located on the torch itself which is mounted on the forward end of the arm, but this has been practically difficult because of the restriction of load weight as noted above. In addition, the previously known wire straightening devices do not attain perfect performance in that they sometimes fail to straighten welding wires perfectly.